The present invention relates to a magnetic resonance imaging (MRI) system for obtaining image data reflecting density and/or relaxation time data of nuclear spins of specific nuclei in an object ( e.g., a living subject or a patient) being examined through utilizing a magnetic resonance (MR) phenomenon.
In an MRI system for, for example, a medical diagnosis, a uniform static field is applied to a subject under examination (i.e. a patient), a linear gradient field is appropriately superposed on the static field, and a high-frequency field acting as an excitation pulse is applied to the patient, to excite an MR phenomenon. MR signals produced by the MR phenomenon are detected and acquired, and acquisition data is subjected to two-dimensional Fourier transformation or other suitable process, in order to obtain tomographic image data of a predetermined portion of the patient.
However, since, in the case of the imaging method used by the conventional system, MR signals from a portion separated from the center of the linear gradient field are acquired as high-frequency signals having a frequency depending on a distance from the center. When these signals are two-dimensional Fourier transformed to be visualized, "aliasing" often occurs in a phase encoding direction for a normal scan operation.
This phenomenon will now be explained in detail, with reference to a case wherein a tomographic image of the patient's head is to be obtained. FIG. 1 shows an example of a tomographic image obtained by reconstruction when a patient is placed in a location separated from the center of an image, i.e., the central position of the linear gradient field by a predetermined value or more. In this case, an image of the patient's nose located near the left end of an image area falls outside the image area, and is appeared on the right end at the opposite side in the image area due to aliasing, as shown in FIG. 1.
This problem can be prevented in such a manner that the center of the patient for which a tomographic image is to be obtained is always located at the center of the linear gradient field. However, the position of the patient relative to the linear gradient field may be limited by the range of movement of the bed on which the patient lies or by the position of a portion of interest to be imaged. Thus, the above problem often cannot be solved.
As another means for solving the above problem, a data sampling interval or a phase encoding pitch is shortened in order to expand the imaging area. However, in order to employ this method successfully and to maintain the spatial resolution at as high a level as that before application of this method, the matrix size of the acquisition data must be increased. Thus, if this method is employed, the scan time (the time required for MR excitation-MR data acquisition) and the reconstruction time (the time required for obtaining an image from the acquisition data) are prolonged, with the result that the storage area necessary for storing data during and data after processing is undesirably increased.